Scientists have discovered that a genetic variant is strongly linked to obesity risk - but only if you're born after 1942.

Previous research on genetics and obesity had linked a variant in a gene called FTO with obesity risk. But although the link was strong in some people, others didn't seem to be affected by it at all. 

Now researchers may have worked out why, after discovering that there may be another factor at play - when someone was born.

In the new study a team from the Massachusetts General Hospital in the US found that there was no correlation between the FTO variant and obesity in participants born before 1942, but the correlation in those born afterwards was twice as strong as previously reported.

The team used data from the Framingham Heart Study, a multi-generational study that has been analysing the cardiovascular health of participants in Framingham, Massachusetts for more than 50 years.

They looked at data collected from more than 5,000 participants between 1971, when the participants were aged between 27 and 63, until 2008. 

"Looking at participants in the Framingham Heart Study, we found that the correlation between the best known obesity-associated gene variant and body mass index increased significantly as the year of birth of participants increased," said lead researcher James Niels Rosenquist in a press release.

"These results - to our knowledge the first of their kind - suggest that this and perhaps other correlations between gene variants and physical traits may very significantly depending on when individuals were born, even for those born into the same families."

During the study, the scientists measured the relationship between participants' body mass index (BMI) and the FTO variant they were born with. They found that there was no link between the two for people born before 1942, but it was twice as strong as previously reported in participants born after that year.

The results have been published in the Proceedings of the National Academy of Sciences, and the authors believe that their findings suggest that environmental factors that came into play after World War II somehow caused the FTO variant to contribute to obesity risk.

Further research is needed on what these environmental factors might be, but the authors believe it could be related to an "increased reliance on technology rather than physical labor and the availability of high-calorie processed foods," as the press release explains.

"We know that environment plays a huge role in the expression of genes, and the fact that our effect can be seen even among siblings born during different years implies that global environmental factors such as trends in food products and workplace activity, not just those found within families, may impact genetic traits," said Rosenquist in the release.

"Our results underscore the importance of interpreting any genetic studies with a grain of salt and leave open the possibility that new genetic risk factors may be seen in the future due to different genetically-driven responses to our ever-changing environment."

Source: ScienceDaily